Current subsea Blow Out Preventer (BOP) testing practice (in U.S. “Oil and Gas Drilling Operation,” Subpart D, 30 C.F.R. Chapter II, current Edition; and generally worldwide) is to view shut-in test pressures on circular chart recorders and wait until a 5-minute period of reasonably stable pressures is obtained (see FIG. 1). Reasonably stable pressures must be greater than or equal to the required test pressure and allow for temperature-related pressure declines. Tests are initiated well in excess of required pressures. A 5-minute period of reasonably stable pressures is required as proof of non-leaking tests since, absent additional analysis, the periods of overtly declining shut-in pressures could be indicative of leaks in the systems. The basic chart recorder used on a majority of drilling rigs today was patented over one hundred years ago (Wittmer, G. X.: “Recording Apparatus for Fluid Meters,” U.S. Pat. No. 716,973).
In the United States under current regulations, subsea BOP tests, recorded on 4-hour 15,000 psi circular charts, are typically ended when pressure decline rates are in the range −4 to −3 psi/min. This is because the pressure trace begins to appear steady once pressure decline rates diminish to the range −4 to −3 psi/min, making this the as-practiced limit of circular chart resolution. Given the subjective nature of visual chart interpretation, tests are sometimes stopped at pressure decline rates as high as −5 psi/min and as low as −2 psi/min. A decline rate of −3 psi/min is representative of a high standard of current testing practice. The pressure at which this occurs is defined as Ps or the “pressure at stabilization.”
Industry trends toward deeper water, synthetic oil-based fluids, and subsurface conditions requiring increasingly higher test pressures all contribute to lengthy delays while waiting for pressures to stabilize during subsea BOP testing. Also, subsea BOP stacks with redundancy of components and use of multiple-diameter drill strings leads to greater numbers of tests that must be conducted.
An investigation of the phenomenon of lengthy subsea BOP testing times (see Franklin, C. M., Vargo, R. F., Sathuvalli, U. B. and Payne, M. L.: “Advanced Analysis Identifies Greater Efficiency for Testing BOPs in Deep Water,” SPEDC [December 2005] 242-250) conclusively attributed the prolonged decay of pressure with time to heating of the test fluids during pressurization followed by cooling of the fluids during shut-in test periods. They proposed that real time digital analysis of the pressure decay could yield large time and cost savings with safety benefits gained through reduced exposure time of personnel to pressurized lines.
FIG. 2 depicts an example of the basic components involved in testing a subsea BOP stack 8. A drill string tool or test plug 9 is lowered into the interior or throughbore of the BOP and it seats at the lower end of the BOP to seal off the well components further down the wellbore. The system is a pressure vessel comprised of the test line 10 from the Cementing Unit (CU) 12 and the drillpipe 14 from the surface 13 of the rig 16 down to the BOP stack 8 at the mudline 20. In this work, the capacity of the BOP pressure vessel is referred to as the “test volume” (i.e. an isolated portion of the throughbore of the BOP). A choke line 24 and a kill line 26 connect the throughbore at the interior of the BOP to the CU 12. The valves (e.g., annular preventers, pipe rams, shear rams, etc.) 22 in the BOP stack are tested in sequence by closing each valve and then pumping fluid from the CU into the test volume until a “target pressure” is reached (i.e. the “pumping phase”). At the target pressure, pumping stops and the pressure in the test volume is monitored until the test is deemed valid (i.e. the “shut-in phase”). In deepwater wells, the duration of the shut-in phase can be as long as 60 minutes when Synthetic Based Muds (SBM's) are used. Pressure testing a BOP with SBM leads to lengthy testing times as a result of pressure/volume/temperature (PVT) influences associated with the fluid properties of SBM. These influences are especially pronounced in deepwater and high-pressure test environments.
In the example of FIG. 3, eight pipe ram tests averaged 53.5 minutes each, four annular preventer tests averaged 16.8 minutes each, and the total shut-in time was 8.25 hours. In the U.S., the ideal combined shut-in time would be one hour given the U.S. Minerals Management Service (MMS) requirement that each of the 12 tests must hold the required pressure for 5 minutes. In this example, an excess of 7.25 hours was expended waiting for pressures to stabilize.
Pressure declines of non-leaking tests may be attributed to cooling of the fluids in the pressurized system:                Surface-temperature fluids are pumped from the CU into the kill and/or choke line(s) to apply elevated pressure to the subsea BOP components being tested (i.e., these fluids are warmer than their surroundings).        Fluids in the kill and/or choke line(s) compress as additional fluids are pumped in (i.e., these fluids are displaced deeper to cooler surroundings).        Fluids in the kill and/or choke line(s) undergo an internal energy rise when they are compressed; this heat of compression causes a slight elevation of fluid temperatures throughout the system.        The pressurized fluids in the kill and/or choke line(s) cool as they lose heat to their surroundings.        Shut-in test pressures decline as the testing fluids cool; the rate of pressure decline is fastest initially when the temperature differences (ΔT) between fluids and surroundings are greatest, and slows as ΔT becomes less.        
Subsea BOP tests tend to take longer with synthetic base muds (SBM) than with water base fluids (see FIG. 4) because:                SBM is more compressible than water, hence more SBM (and heat) is pumped-in to attain a given test pressure.        SBM has greater heat of compression (temperature rise) than water.        SBM has lower heat capacity than water so loses heat more slowly and takes longer to cool.        
The problem of BOP testing has existed for some time. Considerable time and effort is expended each year to perform BOP tests. In spite of this, and with the exception of the earlier work by Franklin, et al., BOP testing schemes have not progressed in a long time. Actually, the problem has become aggravated with the passage of time because each year more and more testing is conducted at higher pressures using the current time consuming processes.